The permanent magnet DC motor has been widely used in fields such as office automatization, household appliance, and automobile electric parts, for it has the advantages of simple structure, reliable electromagnetic properties, and simple maintenance.
Referring to FIG. 8, a traditional permanent magnet DC motor usually uses four permanent magnets 101˜104, fixed symmetrically inside the iron shell or housing of the motor stator to produce four field poles, thus, the number of permanent magnets equals the number of field poles and each magnetic circuit is formed by two magnets in series. Viewed from outside the stator, the orientation of adjacent magnets is opposite. However, this traditional rotor configuration will not be suitable for low power motors with a small size. When using high remanence rare earth magnets, thin permanent magnets can satisfy the magnetic requirement of the low power motor. However, thin magnets, especially magnets having a wall thickness of less than 2 mm, are very delicate and are easy to be damaged during manufacture and assembly. To thicken the magnets in order to avoid being damaged in assembly will result in low utilization and wastage of the magnet material. As there are two pieces of magnet in series in each equivalent magnetic circuit, and the magnetic field is so strong, the stator housing will be saturated. In other words, the magnet material is wasted. Furthermore, adjacent magnets with opposite magnetic poles will complicate the process of assembling the magnets into the housing.
U.S. Pat. No. 4,296,343 discloses an electric motor using two magnets and two inwardly projecting pole pieces of the housing to create four field poles. However, the size of the air gaps between the magnets and the armature is equal to the size of the air gaps formed between the inwardly projecting pole pieces and the armature. The resulting field distribution between the magnet poles and the housing poles is uneven.